You might have activated both the Header Text and the Header Image which makes them overlap with each other. You need to disable the Header Text if you want to use a header image instead. To disable the header text, please follow the steps below:
Log in to your wordpress dashboard. If you are an Affilojetpack member, you can login to your Wordpress site by going to the Affilojetpack members area and clicking on the name of your domain on the right sidebar.